The present invention relates to the field of piezoelectric resonators used in applications where an oscillation frequency must be defined with high stability. This is the case for example with drive oscillators or frequency filters used in radio electricity and telephony.
Such resonators are formed from a monocrystalline material such as quartz, crystallized silica variety, cut in a block with a general parallel-epipedic shape, and having electrodes on two of its opposite faces for applying thereto the required electric voltages.
The frequency of a piezoelectric resonator depends on some of its dimensions, and it is adjusted by mechanical grinding operations in the presence of abrasive materials, in several successive steps, of rough shaping first of all then finishing.
The resonator is then provided with its electrodes, formed very often of thin metal layers deposited on the appropriate faces and, before use, is operated for a given duration in an electric formation or "ageing" circuit.
It has in fact been discovered that its working characteristics, the principal one of which is its oscillation frequency, change slowly at constant temperature during the first period of operation, before stabilizing at a fixed value, following an asymptotic law at this value; by way of order of size, a quartz resonator, oscillating at about 5 MHz, may require about 4000 hours before stabilizing at 1.10.sup.-9 per month of its nominal frequency.
Extensive researches carried out by the Applicant to discover the causes of this phenomenon have shown that the mechanical finishing operations were to a great extent the cause thereof. During the finishing of a quartz, the principal surfaces of the crystal are bruised. The abrasive element tears the surface and creates flakes which are removed by careful rinsing. Nevertheless, there still remain surface crevices and dislocations forming a thin heterogeneous layer which is called "disturbed zone." When this zone is exposed to humidity, erosion of the surface occurs and thus particles of quartz--and abrasive--break off.
In practice, despite a final chemical cleaning in an acid medium, the presence of this disturbed zone causes slow-changing phenomena to be established which explain the change in time of the resonator:
progressive diffusion of the metallic material of the electrodes in the disturbed crystalline network, that of quartz having a relatively penetrable or "open" structure; PA1 penetration of water vapor, atmospheric impurities.
Furthermore, it is to be noted that the machining operations disturb the mechanical equilibrium of the block, by introducing stresses therein, the relaxation of which in the course of time is long.
It thus appears that it is the disturbed zone which is partly responsible for the above-mentioned changing phenomena and that the remedy should be the application of a protecting coating to this zone, welding together the flakes and closing up the cracks or crevices in the dislocated crystalline network. The use of external coatings has not up to present given satisfactory results showing an improvement in ageing.